The present invention relates to a backing sheet or mat and, more particularly, to a flexible backing sheet or mat for use in the exposure of photosensitive materials under vacuum conditions.
Vacuum frames have been employed in the past in the exposure of photosensitive materials, for example in photolithographic, photogravure and screen printing processes. In such vacuum exposure processes, one or more mechanically or photo prepared masks or stencils having an image thereon are placed upon a sheet of photosensitive material, such as a film or plate, and both of these sheets are placed on a flexible blanket in a vacuum frame. The vacuum frame is then closed and a vacuum is drawn on the frame such that the supporting blanket is drawn upward against the mask and photosensitive sheet compressing and flattening the mask and photosensitive sheet between the glass cover of the frame and the blanket.
In order to insure that the mask and the photosensitive sheet are uniformly pressed together and against the glass and completely flattened, and to provide a satisfactory background around the perimeter of the latter mentioned sheets, various sheets of backing material have, in the past, been positioned between the blanket and the photosensitive material.
One such form of backing material has comprised a vinyl material about 30 mils in thickness. The relatively rigid nature of such backing material, however, results in several problems. One problem is the long air evacuation times necessary to completely evacuate the air between the sheets prior to exposure. If exposure takes place prior to complete air evacuation, the likelihood substantially increases that the final product will be distorted or misregistered. Air evacuation is particularly difficult with such prior backing sheets because the sheets themselves are not pervious to air and, during evacuation, the edges of the relatively flexible mask and photosensitive sheets will initially be evacuated, thus effecting a seal around the periphery of the sheets which increases the difficulty in removing the air from the center of the sheets. Where the sheets are particularly large, air evacuation may take upwards of 11/4 minutes for each set up, thus substantially lengthening the time necessary to complete a production order. Moreover, another problem with the aforementioned backing sheets is the difficulty in removing kinks, either in the mask, the film or the backing sheet itself, due to the rigidity of these backing sheets. Finally, because of the vinyl material used in these sheets, they are frequently affected by heat during exposure and become dimensionally unstable. Such instability results in a defective final product and loss of dot registration.
Another form of backing material which has been employed in the past is a polyurethane foam material which is covered on one or both sides with a sheet of frosted plastic material. Likewise, these sheets are subject to kinking of the mask or film, extended air evacuation times, and lack of mechanical stability due to the presence of the middle layer of relatively spongy polyurethane foam.
Various forms of cardboard or chipboard have also been used in the past as backing sheets, but these too suffer from air evacuation problems, dimensional instability, and also from coarseness of surface.
The backing sheet or mat constructed in accordance with the principles of the present invention overcomes these aforementioned problems. In the backing sheet or mat of the present invention, air evacuation from the center of the sheet is substantially improved in both quality and total time for evacuation, thus resulting not only in a substantial improvement in the quality of the finished product, but also in the production rate. Thereby, expensive remakes, labor and material costs and production costs are substantially reduced when using the backing sheet or mat of the present invention. The backing sheet or mat of the present invention is also higly dimensionally stable when subjected to either heat or mechanical forces, thereby substantially improving the quality of the finished product and its dot and image registration. The backing sheet of the present invention actually conforms to and smooths out surface irregularities due to varying layers of film thickness and film kinks to assure total contact and eliminate distortion in the final product, and actually enhances the smoothing and removal of kinks which may be formed in the various materials during air evacuation.
In a principal aspect of the present invention, a backing sheet for use in the exposure of photosensitive materials under vacuum conditions comprises a first layer of porous material adapted to receive the photosensitive materials thereon for exposure, a second layer of flexible, but substantially rigid, material beneath the first layer, and a third layer of a flexible material beneath the second layer, the layers being attached together to form the backing sheet.
In another principal aspect of the present invention, a plurality of apertures extend through the thickness of at least one of the second and third layers and, preferably, through both layers, and over substantially the entire area of the layers.
These and other objects, features and advantages of the present invention will be more clearly understood through a consideration of the following detailed description.